1. Field of the Invention
The present invention relates to a material testing apparatus and more particularly to a material testing apparatus of the type including testpiece holders, a driving unit for actuating the holders to plastically deform testpieces made of plastics or the like material and a measuring unit for measuring the extent of color changing of the deformed testpieces (the extent of whitening in the case of testpieces made of plastics) by using an optical system.
2. Prior Art
As is well known, there takes places color changing in the deformed area on a certain material which is subjected to plastic deformation under the effect of bending or stretching force. This color changing is attributable to changing of the structure in the deformed area.
In the case of plastics the plastically deformed area becomes whitened. It is confirmed that an occurrence of the phenomenon of stress whitening is mainly attributable to craze appearing in the form of a stripe extending along a kind of molecule arrangement line, micro-crack and deterioration of improving agent added to plastics (for instance, butadien based rubber adapted to be added to the matrix to improve impact strength of the latter). In practice, the phenomenon of stress whitening occurs while the above-noted factors are superimposed one above another.
The phenomenon of stress whitening of plastic material is one of the important items which has been elaborately researched as characteristics of the material. A generally known thing is that there is a tendency that the phenomenon of whitening appears more highly with material having better impact strength during plastic deforming and on the contrary impact strength inherent to the material is reduced when the occurrence of the phenomenon of whitening is intentionally inhibited. Thus, research and development work has been conducted to produce a material having a reduced occurrence of the phenomenon of whitening and excellent impact strength in consideration of the abovementioned tendency. In spite of the requirement for the material as mentioned above the current state or research and development activities is practically such that they are oriented to produce material having a natural well balanced between the extent of whitening and impact strength.
For this reason there is the necessity for conducting a variety of experiments for examining the extent of whitening when new material, particularly new plastic material, is to be developed. Further, there is a necessity for obtaining objective and quantitative data as experimental data which is usable for developing a new material.
As for the hitherto known experiments for examining the extent of stress whitening are concerned which were reported in technical papers or like, they deal mainly with experiments for plastically deforming material under the effect of stretching force. However, in the case where deformation of conventional plastic products are studied, it is necessary to examine the extent of whitening caused not only due to tensile deformation but also due to bending deformation. In practice, no research and development work is reported probably because of the difficulty of determining operative conditions or the like in the case of bending deformation of plastic material.
When plastic material is tested to plastically deform it under the effect of stretching force, the results or measurements conducted with respect to the extent of whitening are hitherto visually evaluated by the operator's eyes. For this reason the results of experiments are usually reported by way of photographs printed on technical papers or the like. Obviously, photographs have different optical contrast depending on the printing condition, and the kind of printing paper or the like, resulting in a failure of obtaining objective data. As another evaluating method, a five point evaluating method is also employed which is intended to evaluate the extent of whitening by using a 1 to 5 scale. In this method a certain numeral thus obtained represents the extent of whitening of plastic material. However, a drawback of the method is that exact measurement is difficult because an operator's subjectiveness tends to be added to the results of the experiments.